deploy: Remove deployment bootcsum assertion

deploy: Remove deployment bootcsum assertion

When support for devicetree was added, it created a problem
because old and new ostree versions would compute different
checksums for the "boot data".  The scenario here is:

- Have system with ostree < 2020.4
- Reboot into system with ostree 2020.5
- Try to perform an operation that would retain
  that previous booted deployment (common)

Currently ostree iterates over all the deployments
that will be retained and calls `install_deployment_kernel()`,
even for the booted one (which is a bit silly), but
just to verify that all boot data for the targeted
deployments are installed.

This then re-computes the checksum and we'd trip this
assertion.

In practice though, we don't strictly require them to match;
the only thing that will happen if they don't is that we'll
end up with another copy of the kernel/initramfs - and
that only temporarily until the previous deployment
gets GC'd.

Longer term, I think what we really want to do anyways
is probably closer to like a little ostree repo for `/boot`
so that we can e.g. still hardlink kernels there even if
the initramfs changes, or hardlink both kernel/initramfs
if just the devicetree changes, etc.

Closes: https://github.com/ostreedev/ostree/issues/2154